In modern society, over the past fifty years, there has been a shift from metals to polymers, partly because the latter do not oxidize in the environment. For example, corrosion of metal destroys valuable property. Corrosion is the inevitable reaction of metal to form metal oxide when the metal is exposed to environmental conditions that permit such corrosion. Billions of dollars are spent each year to repair or replace metallic structures because of such corrosion.
The corrosion of iron-containing articles, the natural process normally called “rusting”, has prompted considerable effort to find effective, economical ways to prevent or reduce rust. Coating the surface of the iron-containing article, the process of painting, was first attempted to shield the article from the elements needed for the natural rusting reaction to begin.
Iron-containing articles form the structures that have erected the cities and commercial links between them. Ironwork, for such diverse uses as multi-story buildings, suspension bridges, tunnels beneath a mountain or a river, high tension utility powerlines, fuel storage tanks, the Statue of Liberty, the Eiffel Tower, and reinforcement grids for concrete structures of all types, all require such protection from corrosion.
Conventionally, many types of metals are subjected to treatment as a part of manufacturing process. “Treatment” means the treatment of a metal surface which is not bare metal, and preferably has been previously treated with a conventional conversion coating process. Such conversion coatings are well known and have been described, for example, in Metal Handbook, Volume II, 8th Edition, pp. 529-547 of the American Society for Metals and in Metal Finishing Guidebook and Directory, pp. 590-603 (1972). U.S. Pat. No. 4,376,000 (Lindert) describes the process of conversion coating as                1. Cleaning;        2. Water rinsing;        3. Formation of the conversion coating by contact with a suitable phosphate, chromate, or similar conventional bath;        4. Water rinsing;        5. Applying a treatment solution; and        6. Optionally, drying the surface.        
Suitable conversion coatings include, without limitation, iron phosphate, manganese phosphate, zinc phosphate, zinc phosphate modified with calcium, nickel, or manganese ions. For environmental reasons, chromate coatings are less preferred.
Examples of suitable metal surfaces for treatment with conversion coatings include zinc, iron, aluminum and cold-rolled, polished, pickled, and hot-rolled steel and galvanized steel surfaces.
Conventionally, modern steel treatment includes a coating at the time of manufacturing of a phosphate-containing material to form an iron phosphate coating on the surface of the steel. The treatment is often an integral process of steel manufacturing. An example of the art of steel treatment with phosphate materials is U.S. Pat. No. 4,132,572 (Parant et al.) which discloses steel, aluminum and aluminum alloys, zinc and zinc alloys treated to improve corrosion resistance with an aqueous solution of a fluorophosphate salt to passivate the metal surface, before or after phosphatizing and prior to painting. Other examples of commonly practiced zinc phosphate and iron phosphate coating processes and compositions can be found in U.S. Pat. Nos. 3,333,988; 3,297,494; 3,425,876; 3,520,737; 3,101,286; 2,987,428 and 3,129,123.
Other anti-corrosion mechanisms have taken advantage of the Galvanic Series, whereby a less noble metal is a sacrificed in the environment where the iron would otherwise rust. This “cathodic protection” of metal has spawned an enormous industry dedicated to preserving metallic property against the ravages of the environment.
Cathodic protection utilizes the physics of a galvanic circuit, which can be assisted by power to be an active circuit to drive the corroding effects away from the metal being protected or which can be passive without power. Examples of passive galvanic circuitry are disclosed in U.S. Pat. No. 5,650,060 (Huang et al.) for an electrode-based system and in U.S. Pat. No. 5,976,419 (Hawkins et al.) for a coating-based system. Both types of systems rely on a more anodic metal in the Galvanic Series, such as zinc, to protect the more valuable iron in the structure. In the Huang et al. electrode, the zinc is in the form of plate adhered by an ionically conductive adhesive to a structure. In the Hawkins et al. coating, the zinc is in the form of particles dispersed in the binder and inherently conductive polymer. In both cases, the zinc is the anode of the galvanic circuit. The anodic zinc is sacrificed to preserve the cathodic iron.
The combination of inherently conductive polymer with sacrificial metal particles becomes a cathodic protection coating to protect metal substrates in which the metal particles are less noble than the metal of the substrate. Such cathodic protection coating is disclosed in U.S. Pat. No. 6,627,117 (Geer et al.), the disclosure of which is incorporated by reference herein. Such cathodic protection coating is marketed by PolyOne Corporation under the brand Catize® coating.
It has been thought previously that the cathodic protection coatings, such as taught by Hawkin et al. and Geer et al. were required to be applied directly to the surface of untreated steel. This belief caused concern by the users of cathodic protection coatings, because such coatings were often desired to be applied to specific metallic articles, well after the time that such metal was manufactured at the steel plant. Either the cathodic protection coating needed to be applied at the steel plant in substitution for the phosphate treatment or the steel was transported to a customer without the phosphate treatment. The former opportunity disrupts a well-organized steel manufacturing process; the latter exposes the steel to rusting conditions during its time in transit and inventory. Neither arrangement has been satisfactory to users of steel and other metals susceptible to corrosion.